Criminals are exploiting an eBay security weakness that could result in shoppers getting redirected to a malicious webpage that tries to steal bank account information.

The British Broadcasting Corp. (BBC) first reported the scam, saying it had identified more than 100 listings that sent eBay customers to official-looking pages that also asked for usernames and passwords.

EBay has more than 700 million items listed on the site, so the percentage of malicious listings is small. Nevertheless, they are enough of a threat that users of the site should look carefully at the pages they are directed to, researchers say.

EBay acknowledges that the site is vulnerable to so-called cross-site scripting (XSS), one of the most common types of websites attacks.

"Cross-site scripting, carried out by malicious individuals, is an issue affecting sites across the Internet," eBay spokesman Ryan Moore, said in an emailed statement. "This is not a new type of vulnerability on sites such as eBay."

XSS is a method of injecting malicious code into a webpage through the fields in which users type information. Attackers will use the technique to exploit the Javascript and Flash eBay lets sellers write to make their stores more attractive to buyers.

"The criminals behind cross-site scripting and phishing activity intentionally adapt their code and tactics to try to stay ahead of the most sophisticated security systems," Moore said. "Cross-site scripting is not allowed on eBay and we have a range of security features designed to detect and then remove listings containing malicious code."

Nevertheless, the site has had a problem with XSS at least since February, the BBC said Monday.

Because eBay is balancing security with giving sellers design freedom, the risk of XSS attacks will be greater on the site then on online marketplaces that don't have to make such tradeoffs, Troy Hunt, a website security specialist, said.

"EBay is in a tricky position as they want to give customers design freedom over their listings," Hunt said. "Because this freedom extends to modifying the markup of the page, those with malicious intent are always going to have an avenue that's at some risk to XSS."

To reduce the effectiveness of XSS attacks, he suggests a whitelist approach to the use of Javascript, so only preapproved operations are allowed. Currently, eBay takes a blacklist-like approach where it prevents only certain uses of the technology.

"That model is fundamentally flawed," Hirvonen said. "Somebody is always going to find a way of doing one of those things that they (eBay) are trying to deny."

Another option would be for eBay to provide the Javascript code to sellers. While that would greatly reduce the level of customization, that approach would be more secure, Hirvonen said.

This story, "Is eBay Trading Too Much Security for Seller Happiness?" was originally published by
CSO.